问题
单项选择题
A stock priced at $100 has a 70 percent probability of moving up and a 30 percent probability of moving down. If it moves up, it increases by a factor of 1.02. If it moves down, it decreases by a factor of 1/1.02. What is the probability that the stock will be $100 after two successive periods（）
A. 9%.
B. 42%.
C. 21%.
答案
参考答案：B
解析：
For the stock to be $100 after two periods, it must move up once and move down once: $100×1.02×(1/1.02)=$100. This can happen in one of two ways: 1 ) the stock moves up during period one and down during period two; or 2) the stock moves down during period one and up during period two. The probability of either event is 0.70×0.30=0.21. The combined probability of either event is 2 (0.21)=0.42 or 42%.